This invention relates to semiconductor lasers and, more particularly, to quantum effects in heterostructure lasers.
A conventional double heterostructure (DH) junction laser is generally fabricated from the GaAs-AlGaAs materials system and includes a pair of wide bandgap layers (e.g., AlGaAs) of opposite conductivity type and an active region (e.g., GaAs) sandwiched therebetween. The interfaces between th active region and the wide bandgap layers form a pair of heterojunctions which provide both optical and carrier confinement. For C.W. operation at room temperature the thickness of the active region is between about .lambda./2 and 1.0 .mu.m, where .lambda. is the wavelength of the laser radiation as measured in the semiconductor. In practice, it has been found that this thickness is preferably between about 0.2 and 0.5 .mu.m for GaAs-AlGaAs DH lasers.
The lasing threshold of conventional DH lasers is a function of the thickness of the active region. As this thickness decreases so does the threshold until at some point, about .lambda./2, the waveguide formed by the heterojunctions fails to provide adequate optical confinement. Thereafter, the threshold begins to increase.
On the other hand, the wavelength of radiation generated by conventional DH lasers is determined by the bandgap of the active region. That is, radiative recombination of electrons in the conduction band with holes in the valence band produces the laser light. It is known that the wavelength of the radiation can be changed by altering the composition of the active region. Thus, a p-type GaAs active region produces radiation at about 0.9 .mu.m. When aluminum is added to the active region, the wavelength shifts to as low as 0.77 .mu.m.
From the standpoint of quantum effects, because the active region of conventional DH lasers is thicker than 500 Angstroms (typically 2000 to 5000 Angstroms as discussed above), the discrete energy levels associated with confined electrons are so closely spaced that quantum effects are negligible.